Compression release-type engine brakes are well-known in the art. Engine brakes or retarders are designed to temporarily convert an internal combustion engine of either the spark ignition or compression ignition type into an air compressor. The fundamental braking power is achieved by preventing fuel injection during the compression stroke of a piston, compressing the captured air mass, and releasing the compressed air at or near a top-dead-center position of a piston into an exhaust manifold. The energy expended in compression release braking systems is controlled, for the most part, by the volume of gas compressed, the timing of the release of the gas into the exhaust manifold and the amount of gas released. A compression release brake decreases the kinetic energy of an engine by opposing the upward motion of the engine's pistons on the compression stroke. As a piston travels upward on its compression upstroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. When the piston nears the top of its stroke, an exhaust valve is opened to "release" the compressed gases. The pressure having been released from the cylinder, the piston cannot recapture the energy stored in the compressed gases on the subsequent expansion downstroke. In so doing, the engine develops retarding power to help slow down the vehicle. This provides the operator with increased control over the vehicle.
A properly designed and adjusted compression release-type engine retarder can develop retarding power that is a substantial portion of the power developed by the engine on positive power. Compression release-type retarders of this type supplement the braking capacity of the primary vehicle wheel braking system. In so doing, these retarders may substantially extend the life of the primary wheel braking system of the vehicle.
The basic design of a compression release type engine retarding system is disclosed in U.S. Pat. No. 3,220,392 to Cummins, which is incorporated herein by reference. The compression release-type engine brake disclosed in the Cummins patent employs a hydraulic control system to operate the exhaust valves to effect the compression release event. The hydraulic control system engages the engine's existing valve actuation system, namely, the rocker arms of the engine.
When the engine is operating under positive power, the hydraulic control system of the compression release retarder is disengaged from the valve control system, so that no compression release event occurs. When compression release retarding is desired, the engine is deprived of fuel and the hydraulic control system of the compression release brake engages the valve activation system of the engine. The valve activation system drives the compression release brake to produce compression release events at the appropriate times.
Typically, it is desirable to use the compression release-type engine retarder to open an engine exhaust valve as late in the engine cycle as possible. In this way, the engine develops greater compression, allowing more energy to be dissipated through the compression release retarder. Delaying the opening of the exhaust valve in the compression release event, however, may substantially increase the loading on critical engine components. The force required to open the exhaust valve during the compression release event is transmitted back through the hydraulic system.
In a compression release engine retarder it is desirable to provide accurate timing of exhaust valve opening. To this end, it is advantageous in these systems to apply sharp hydraulic pulses to the slave pistons so that they open the exhaust valves rapidly. In order to both stop the slave pistons' motion and prevent excessive opening of the associated exhaust valves, stroke-limiting mechanisms have been employed to reduce the hydraulic fluid pressure when either the hydraulic fluid pressure reaches the predetermined maximum or the slave pistons have reached the end of their desired stroke. The term stroke-limiting generally refers to modification of the forward motion of the slave piston in order to limit the total travel of the slave piston or to reduce the length of the slave motion event. The disadvantages of excessive slave piston travel include excessive exhaust valve travel and possible contact of exhaust valves with the engine piston, increased overall braking apparatus and engine height, and overtravel of the slave piston return spring.
In the present invention, Applicants have designed an innovative slave piston for a common rail, variable valve actuation system. The present invention has been designed to overcome limitations in stroke-limiting and lash adjustment design found in the prior art.
There are several categories of stroke-limiting designs for common rail variable valve actuation systems. One design relies on precise trigger valve timing to avoid valve-to-piston contact. This design may be unacceptable with respect to potential failure modes. Other designs that use hard stops with oil squeeze films may have difficulty meeting long range durability requirements, considering start-up conditions when there could be insufficient oil in the squeeze film. Still other designs that limit the stroke only by bleeding high-pressure oil behind the slave piston have excessive oil utilization and require an unacceptable increase in the capacity of the high-pressure pump. Other designs that employ a separate stroke-limiting piston in addition to the slave piston are excessively complex. Additionally, designs having an occluding orifice on the slave piston side of the stroke-limiting piston are not fail-safe with respect to degradation of the flow metering edges. Designs that cut off the flow only to the slave piston, such as U.S. Pat. No. 5,531,192 assigned to Caterpillar, are not suitable for decompression braking because valve-to-piston contact could result from entrained air or check valve failure. The present design avoids the risk of valve-to-piston contact due to entrained air or failure of a check valve.
The electronically-controlled, common rail, decompression braking system of the present invention provides variable timing of the opening of the engine exhaust valve to optimize retarding power. The present invention comprises a high-pressure common rail, high-speed electronic trigger valve, means of stroke limiting, and slave piston positioned over the engine exhaust valve or cross head. Opening the trigger valve routes high-pressure hydraulic fluid to a plenum above the slave piston, which displaces the slave piston and opens the exhaust valve. The displacement of the engine exhaust valve must be limited to avoid valve-to-piston contact. Closing the trigger valve connects the slave piston plenum to drain pressure, which causes the slave piston and the exhaust valve to close.
The present invention is directed to a stroke-limiting slave piston design in which the travel of the slave piston is limited by dropping the pressure above the slave piston to drain combined with the force of the exhaust valve and slave piston springs. Dropping the slave piston pressure to drain while the trigger valve is open is accomplished by closing the port between the slave piston plenum and the trigger valve while opening a port between the slave piston plenum and drain. The port flow areas are defined by the mating of grooves, circumferentially-arranged holes, or slots in the slave piston and slave piston cylinder. The flow area versus piston displacement characteristics are built-in and fail-safe. They are defined to provide acceptable valve overshoot, pressure spikes, and utilization of high-pressure oil. Opening the passage to drain slightly before the high-pressure flow is completely cut off provides significant advantages compared to prior art with respect to potential failure modes, tolerance to entrained air in the hydraulic fluid, risk of cavitation damage, design simplicity, and cost. Positioning the lower flow port in the slave piston cylinder so that the lower flow port opens before or at approximately the same piston displacement as the upper flow port is completely occluded serves to eliminate the check valve, which is otherwise needed to ensure that the piston does not get stuck at a displacement at which the upper flow port is fully occluded. There is also a reduction in component cost from elimination of the check valve.
The flow area of the upper and lower ports is designed to vary as a function of slave piston longitudinal position in order to provide: rapid opening of the engine valve, minimal overshoot of the engine valve relative to the desired maximum stroke, acceptable peak pressure in the fluid line between the trigger valve and the upper port, acceptable time for the slave piston to return to its initial longitudinal position after the trigger valve is switched to connect the upper port to drain, and acceptably low average flow of high-pressure oil from the common rail. These flow area profiles may be implemented by the mating of one or two annular channels (or undercuts) with a plurality of circumferentially-arranged holes or slots in the slave piston or slave piston cylinder. In a preferred embodiment, there is a single annular channel (or undercut) on the slave piston and, for both the upper and lower ports, a plurality of circumferentially-arranged holes or slots in the slave piston cylinder (sleeve). An alternative embodiment may have annular channels (undercuts) in the slave piston cylinder for both the upper and lower ports and a plurality of circumferentially-arranged holes or slots in the slave piston.
The present invention includes a means for setting the slave piston lash without affecting the built-in slave piston stroke. This independent lash adjustment means may comprise a sleeve threaded into the brake housing, which is bolted to the engine, to allow lash adjustment and additionally, some means, such as a locking ring, of ensuring that the position does not change as a result of vibration and cyclic loading. The sleeve may be made of steel. The connection to the upper port may be sealed on either side by O-rings or other seals around the sleeve to prevent leakage when the trigger valve is positioned to connect the upper port to the high-pressure common rail. In a preferred embodiment, there are holes in the sleeve connecting the volume between the top of the slave piston and the sleeve with the volume between the top of the sleeve and the housing bore in which the sleeve is disposed. The purpose of these holes is to transmit the load required to open the engine valve to the housing. The sleeve is designed to be partially pressure-balanced to reduce the forces on the threads and so that the net pressure force will not act in a direction which could loosen the locking ring. An advantage of a steel sleeve design is that the edges which define the flow areas which provide the stroke limiting function are more durable than the iron casting of the housing. An optional element is a stroke-adjustment screw in the top of the sleeve.
An alternate design of the present invention provides a hydraulic lash adjusting feature. The independent lash adjustment means may comprise a fixed sleeve provided in the brake housing, a hydraulic lash adjustor piston, an additional plenum between the hydraulic lash adjustor piston and the slave piston, additional hydraulic lash adjustor springs to urge the hydraulic lash adjustor piston upward, and a check valve in the oil supply line to retain the oil in the lash adjustor plenum when the engine exhaust valve is opened by the slave piston. The lash adjustor may be supplied by engine lubricating oil switched by a control valve, which is open when the engine retarder is switched on. When this control valve is open and the engine exhaust valve is closed, engine lubricating oil flows into the lash adjustor plenum and urges the lash adjustor piston down, taking up the lash in the valve train. The lash adjustor is designed so that the engine lubricating oil supply pressure will overcome the lash adjustor spring but will not overcome the engine valve springs. When the engine exhaust valve is opened by the slave piston, the hydraulic lash adjustor check valve retains the oil in the hydraulic lash adjustor plenum. The hydraulic lash adjustor piston must be sized large enough to handle the high loads associated with opening the exhaust valve near top center of the engine compression stroke for engine retarding. When the engine retarder is switched off, the control valve in the supply line to the hydraulic lash adjustor is closed, and oil leaks slowly out of the hydraulic lash adjustor plenum due to the force of the hydraulic lash adjustor springs.
The present invention limits motion in a common rail, variable valve actuation system to avoid contact between the engine valve and engine piston. The most stringent requirements for stroke limiting are for decompression braking where a large force is required to open the engine exhaust valve, the cylinder pressure force is reduced to nearly zero as the exhaust valve is opened, and the exhaust valve is opened near top-dead-center. The market demands that such a design must be fail-safe, durable, and low cost. The present invention meets these needs and provides other benefits as well.